An evaluation of the influence of water depth and river inflow on quantitative Cladocera-based temperature and lake level inferences in a shallow boreal lake

Hydrological parameters can potentially have an overwhelming influence on sedimentary assemblages of Cladocera at certain sampling sites that can cause problems for palaeoenvironmental reconstructions. We applied a previously developed Cladocera-based inference model of water depth and a mean July air temperature transfer function developed in this study to a surface sediment dataset of fossil Cladocera from a lake in eastern Finland aiming to investigate the influence of stream flow and water depth on reconstruction results. The developed temperature-inference model, using the weighted averaging-partial least squares technique, had relatively favourable performance statistics suggesting that it is valid in means of performing temperature estimations. When the temperature model was applied to the intralake samples, the lotic samples had inferred values mostly within the model??s prediction error and only one lotic sample showed an underestimated temperature. Samples taken from depths over ~3 m inferred generally underestimated temperatures, although most of the values were within the model??s prediction error. The water depth reconstructions correlated significantly with the measured water depth, but the shallowest samples and most of the lotic samples yielded overestimated inferred values and the samples taken from depths >5 m showed underestimated values. In both reconstruction sets, the inferred values were underestimated in samples taken from deeper sites. Based on the present results, it may be recommendable that downcore sediment samples should be taken from intermediate depths, where also the diversity is higher, and deepest sites and inflows should be avoided. However, more research is needed to validate these results in a larger geographical context.     

Diatom assemblages in sediments of Lake Juusa, Southern Estonia with an assessment of their habitat

Diatom analysis of surface sediments and two sediment cores from different sedimentation areas of a small closed lake was undertaken with the aim of acquiring knowledge on the dependence of the distribution of diatom assemblages on lake bathymetry. Lake Juusa was selected for the study because we have for this lake a large data set about the lithological composition of sediments and macrofossil and cladoceran records for the Holocene. A high carbonate content (20–60%) in the sediment sequence indicates high carbonacity and relatively stable pH values during the Holocene. On the basis of comprehensive analysis, abrupt water-level fluctuations and changes in the trophic status were established. Results of this study showed that the fluctuations of the water-level were the leading factor determining the habitats of diatom assemblages in the lake. In the surface sediment samples planktonic species such as Cyclotella spp., Stephanodiscus spp. and Aulacoseira spp. had a depth optimum at 3–4 m and the most abundant periphytic taxa were distributed mostly at depths shallower than 3.5 m. The same regularity was established in sediment cores where a good correlation between planktonic species and lake water depth was found in sediments accumulated at water depths >4 m. Lake Juusa appears to be a proper site for detailed environmental reconstructions over the Holocene, and the results will give us a good opportunity to analyse the history of water-level fluctuations in other small Estonian lakes. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: K. Martens     

Climate-related changes during the Late Glacial and early Holocene in northern Poland, as derived from the sediments of Lake Sierzywk

Reconstruction of past climate change and ecosystem response is important to correctly assess the impacts of global warming. In this study, we provide a paleoenvironmental record of in-lake and catchment changes in northern Poland during the Late Glacial and early Holocene using various biotic proxies (pollen, macrofossils and Cladocera) preserved in the lake sedimentary record. Chronology was derived from palynological correlation with a well-dated pollen sequence from nearby-lying Lake Ostrowite and some well-dated events of vegetation history in Central Europe. Pollen analysis provided information on regional climate change affecting vegetation dynamics, whereas macrofossils supplied substantial information on the response of local flora and fauna to climatic, geomorphological and limnological changes. Data were supplemented by analysis of Cladocera remains, which are of special importance because of their quick response to changes in trophic conditions and climate (especially temperature). The bottom of the sediment core reflects an initial stage of the lake formed during the late Aller?d. The Younger Dryas cooling apparently resulted in forest recession and presence of cold tolerant Cladocera species. Due to amelioration of climate at the end of the Younger Dryas and melting of ice, the lake deepened. The beginning of the Holocene was characterised by forest shrinkage and induced clear changes in local flora and fauna communities. The regional vegetation development deduced from the lake’s core is generally consistent with the vegetation history of central Europe. Due to the location of the site near the seashore (oceanic climate and western wind), signals of warming came earlier than inland and in eastern Poland.     

Reconstruction of palaeoecological and palaeoclimatic conditions of the Holocene in the south of the Taimyr according to an analysis of lake sediments

A sediment core from Khatanga-12 Lake (Taimyr Peninsula, Krasnoyarsk krai) has been studied. The 131.5-cm-long core covers ca. 7100 years of sedimentation. Chironomid analysis, a qualitative reconstruction of the paleoenvironment in the region, and a quantitative reconstruction of variations of the mean July air temperature and in the water depth of the lake have been performed using Northern Russia chironomid-inferred mean July temperature models (Nazarova et al., 2008, 2011, 2015). Khatanga-12 Lake was formed during the Middle Holocene warming as a result of thermokarst processes. The development of the lake ecosystem at different stages of its development was influenced by climatic and cryolithogenic factors. The Middle Holocene warming, which occurred around 7100–6250 cal. years BP, activated thermokarst processes and resulted in the formation of the lake basin. Later, between 6250 and 4500 cal. years BP, a period of cooling took place, as is proved by chironomid analysis. The bottom sediments of the lake during this period were formed by erosion processes on the lake shores. The reconstructed conditions were close to the modern after 2500 cal. years BP.     

Lake–peat bog transformation recorded in the sediments of the Stare Biele mire (Northeastern Poland)

The history of Stare Biele paleolake (northeast Poland) has been reconstructed using subfossil Cladocera remains and pollen and spores of aquatic and mire plants from a sediment core. Sediment accumulation began approximately 12,000 years ago during the Older Dryas chronozone. Throughout the entire Late Glacial period, the basin was a small, low-trophic state lake with a developed open-water zone. A well-recorded increase in the trophic state started at the beginning of Holocene. The lake reached its highest trophic level during the early and middle Atlantic chronozone. The first human activity in the lake catchment area occurred at this time, as recorded by fern spores and numerous charcoal grains. Repeated rises in lake water level are documented at the beginning and throughout the early part of the Younger Dryas. Two clear events of decreasing lake water levels are recorded, first during the middle part of the Younger Dryas and second in the Preboreal. Terrestrialization processes first intensified at the end of Atlantic period, which appears to correspond to a decrease in pH. Guest editors: K. Buczkó, J. Korponai, J. Padisák & S. W. Starratt Palaeolimnological Proxies as Tools of Environmental Reconstruction in Fresh Water     

Reconstruction of the history of a thermokarst lake in the Mid-Holocene based on an analysis of subfossil Cladocera (Siberia,Central Yakutia)

This paper presents the results of a cladoceran analysis of a sediment core with a lenght of 382 cm collected from a pingo in alas Khara Bulgunnyakh located in Central Yakutia. Based on the Cladocera analysis results, the formation of the lake had started during the Holocene climatic optimum ~6600 cal. yrs. BP. The analysis of changes in the species composition of subfossil cladoceran communities made it possible to identify, on the basis of cluster analysis, four statistically significant ecological zones. The period of optimal conditions for the Cladocera community is defined by a complex evened community structure and numerous cladoceran remains in the sediments accumulated between 6500 and 6000 cal. yrs. BP. The history of the thermokarst water body development and existence reconstructed on the basis of the subfossil cladoceran analysis was rapid and short.     

History of the zoocenosis of Lake Glubokoe according to animal remains in the bottom sediments

The remains of all the groups of invertebrates in a 131 cm long sediment core taken from the main basin of the lake and corresponding to about 600 years have been examined and counted. The middle horizons (90–40 cm) are dominated by Testacida. Deeper layers and those closer to the surface are dominated by Cladocera. Among the Cladocera, planktonic forms dominate at all depths of the sediment. The present zoocenosis has existed for about 400 years. A 50 cm core from the lake bay has also been studied. Cladocera dominate at 50–40 cm and near the surface. Porifera spicules are abundant at 20–10 cm while Protozoa are numerous at 30 cm. Among the Cladocera, planktonic forms are less important than in the main lake basin.     

Late glacial and Holocene environmental changes inferred from sediments in Lake Myklevatnet,Nordfjord, western Norway

Late Glacial and Holocene environmental changes were reconstructed using physical, chemical and biological proxies in Lake Myklevatnet, Allmenningen, (5º13′17″E, 61º55′13″N) located at the northern side of Nordfjorden at the coast of western Norway. Myklevatnet (123 m a.s.l.) lies above the Late Glacial marine limit and contains sediments back to approximately 14,300 years before a.d. 2000 (b2k). Because the lake is located ~48 km beyond the margin of the Younger Dryas (YD) fjord and valley glaciers further inland, and did not receive glacier meltwater from local glaciers during the YD, the lake record provides supplementary information to Lake Kråkenes that received glacial meltwater from a local YD glacier. Lake Myklevatnet has a small catchment and is sensitive to Late Glacial and Holocene climate and environmental changes in the coastal region of western Norway. The age-depth relationship was inferred from a radiocarbon- and tephra-based smoothing-spline model with correlated ages from oxygen isotope maxima and minima in the Late Glacial sequence of the NGRIP ice core (in years b2k) to refine the basal chronology in the Myklevatnet record. The results indicate a two-step YD warming, colder early YD temperatures than in the later part of the YD, and considerably more climate and environmental variability during the late Holocene in western Norway than recorded previously in the oxygen isotopes from Greenland ice cores. The Myklevatnet record is also compared with other Late Glacial and Holocene terrestrial and marine proxy reconstructions in the North Atlantic realm.     

Eutrophication,recovery and temperature in Lake Mjøsa: detecting trends with monitoring data and sediment records

1. How climate warming may interact with other pressures on aquatic ecosystems is an important issue for research and management. We combined lake monitoring data with a palaeolimnological study to explore the combined effects of eutrophication and subsequent oligotrophication with a long‐term temperature increase in epilimnetic waters. Our goals were (i) to evaluate how well sediment‐based reconstructions reflect the instrumental observations, (ii) to use the palaeo‐record to characterise a reference state for the lake and (iii) to explore whether data from the sediment record can aid in separating the effects of nutrient load and temperature in a large and deep lake. 2. Lake Mjøsa is a large and deep lake in south‐eastern Norway. Eutrophication symptoms peaked in the 1970s, which led to extensive measures to reduce the phosphorus load. A monitoring programme has run continuously from 1972. Monitoring has documented a marked decrease in phosphorus load and algal biomass and also revealed an increase in epilimnetic temperature and extended summer stratification. 3. Records of algal pigments and diatoms were extracted from sediment cores taken from 236 m depth. The pigment record documented dramatic changes in lake production consistent with the monitoring record. The diatom record reflected well the eutrophication history of the lake and also demonstrated that the assemblage of the recent recovery stage differs from that of the pre‐eutrophication period. 4. Ordination of diatom assemblages over time constrained by proxies for nutrient load and temperature indicated that the diatom assemblage correlated with both factors, which together accounted for 60% of the variation in diatom composition. No interaction was detected between these factors. The results suggest that the diatom assemblage has responded to varying nutrient loads as well as to changes in temperature and/or factors that correlate with temperature. 5. Reconstructions of algal biomass and total phosphorus content mirrored known changes through the monitoring period, although the absolute phosphorus estimates were too high relative to the instrumental record. The sediment record from Lake Mjøsa provides a baseline for lake production in terms of algal pigments and organic contents, and for the diatom assemblage composition in a pristine stage.     

The long-term succession of cladoceran fauna and palaeoclimate forcing: A 14,600-year record from Plešné Lake,the Bohemian Forest

A sediment record of cladoceran remains was analysed in a 543 cm long core from Ple?né jezero (Ple?né Lake), the Bohemian Forest, Czech Republic. The core covered the time period from the Oldest Dryas to the present. Littoral and benthic Cladocera included 11 species of the family Chydoridae while three species (Bosmina longispina, Daphnia cf. pulicaria and D. cf. longispina) lived in the open water. Remains of Alona quadrangularis and Chydrous sphaericus occurred in the oldest sediment layers from the beginning of the Bølling chronozone. Bosmina longispina and Daphnia cf. pulicaria appeared about 400 years later. Inorganic sediment accumulated at a relatively high rate of ～ 90 mg cm^?2 yr^?1 at that time, diluting cladoceran remains and organic matter. Remains of Cladocera accumulated at 0.1 to 0.01 of the Holocene rate, making it difficult to observe effects of climate variation on the species structure of Cladocera in the Late-Glacial. Production of remains increased after warming during the Younger Dryas-Preboreal transition at ～11.6 kyr BP, and the proportion of littoral species increased. The most important change in cladoceran fauna occurred at ～10.5 kyr BP and culminated with afforestation of the catchment around 10.3 kyr BP. The domaination of Bosmina longispina lasted for ～250 years. The afforestation occurred concurrently with a decrease in lake water pH. Bosmina longispina and Daphnia cf. pulicaria disappeared, production of cladoceran remains decreased, but biodiversity increased. Planktonic Cladocera were represented by Daphnia cf. longispina during most of the rest of the Holocene. The production of Cladocera never reached the Preboreal level. Since ～ 5 cal. kyr BP, the inferred pH continuously decreased. The final decline was likely caused by cooling during the Little Ice Age and by sulphur emissions from ore smelting. The recent acidification of lake water and impoverishment of aquatic fauna was brought about by emissions of sulphur and nitrogen compounds in the 20^th century.     

Transition from shallow lake to a wetland: a multi-proxy case study in Zalavári Pond,Lake Balaton,Hungary

Lake Balaton, the largest shallow lake in Central Europe, has no natural outlet, therefore, underwent water level changes during its 15,000–17,000 years of history. The lake is very sensitive to both climate changes and human impacts. Surroundings have been inhabited since the Stone Age; however, heavy human impact can be recognized during the past 6000 years. In this study, we established three different stages for and reconstructed water level changes of Lake Balaton by geochemical data, subfossil Cladocera and diatom remains in the sediments of the Zalavári Pond, a part of the Kis-Balaton wetland. In 9900–8600 cal. year BP, climate was dry, water level was low, and there was a wetland in this area. Although organic matter content was low in the sediment, the ratio of Fe/Mn was high. Between 5600 and 5000 cal. year BP, water level increased, Fe/Mn ratio shows that oxygen conditions of sediments was improved in agreement with the relatively low number of diatom remains and dense chydorid remains. About 5000 cal. year BP, water level of Lake Balaton decreased as indicated by high organic content with low carbonate and high Fe/Mn ratio in the sediments (oxygen depletion). At the bottom of this section, high Fe and S concentrations showed accumulation of pyrite (FeS₂) that is common in wetlands with very low redox potential. Low abundance of Cladocera remains together with rich and diverse diatom flora confirm the low water level hypothesis. Our data support that the water level of Lake Balaton was higher between 8600 and 5000 cal. year BP than it is at present.     

Cladocera paleocommunity to disentangle the impact of anthropogenic and climatic stressors on a deep subalpine lake ecosystem (Lake Iseo,Italy)

In big lakes with strong anthropogenic pressure, it is usually difficult to disentangle the impacts of climate variability from those driven by eutrophication. The present work aimed at the reconstruction of change in the species distribution and density of subfossil Cladocera in Lake Iseo (Italy) in relation to climate and anthropogenic pressure. We related subfossil Cladocera species composition and density in an 80-cm sediment core collected in the pelagic zone of Lake Iseo to long-term temperature trends and phosphorus concentration inferred by diatoms frustules. The Cladocera remains detected in Lake Iseo sediment reflected the species composition and density of modern pelagic Cladocera assemblages. Cladocera rapidly respond to environmental change, and that climate change combined with eutrophication can induce changes in community composition and species density. At the beginning of twentieth century, when global warming was not yet so accentuated, the nutrient increase in water resulted as the principal driver in determining the long-term development of plankton communities and pelagic food web structure. Moreover, catchment-related processes may decisively affect both species composition and density of the lake planktonic communities due to the decrease of lake water transparency induced by input of inorganic material from the catchment area to the lake. The paleolimnological investigation, through the combined study of biotic and abiotic factor, allowed clarifying the synergic effects of the most important drivers of change in lake ecosystems, suggesting that climatic factors should be considered with nutrient availability as determinant element in controlling the temporal development of plankton communities and pelagic food web structure.

     

真光层深度的遥感反演及其在富营养化评价中的应用

真光层深度直接影响水体中浮游植物的分布和初级生产力以及水体生态环境,是水生态研究的一个重要参数.利用2006年10月24日～11月2日太湖水下实测光谱数据和光合有效辐射(PAR)数据,通过数据的处理和分析,尝试建立真光层深度与水面以下遥感反射率的关系模型,并利用真光层深度与透明度的关系,建立水体富营养化真光层深度评价模型.研究结果表明:真光层深度与归一化遥感反射率具有很好的相关性;选用特定波段的归一化反射率作为变量,建立两者的关系模型能较好的反演真光层深度,所建立的模型算法中,指数模型拟合方程的综合效果好于其他模型,波段比值算法反演精度要好于单波段算法;利用利用真光层深度进行富营养化评价具有一定的应用价值,利用该模型对太湖水体进行富营养化评价,得出太湖西部湖区大部分已富营养化,东部湖区处于中营养化和轻度富营养化状态.     

Zooplankton (Cladocera) in assessments of biologic integrity and reference conditions: application of sedimentary assemblages from shallow boreal lakes

Zooplankton are potentially powerful proxies for the assessments of biologic integrity. The paleolimnological perspective and use of fossil Cladocera also provide the means to reconstruct reference conditions and natural long-term community dynamics. Unfortunately, the use of zooplankton in lake quality assessments is currently underexploited. We studied a surface sediment dataset of 41 lakes in Finland to examine the relationship between Cladocera remains and environmental variables. Of the examined environmental variables, total phosphorus availability was found to be the most important variable in explaining the Cladocera community composition. Following the tests on species environment relations, we selected a lake trophic typology as the most suitable environmental variable for developing a new tool for limnoecological quality assessments. A test of the model on a modern and historic sample from a eutrophied lake showed that the test lake has proceeded from “mesotrophic/poor” to “eutrophic/bad” limnoecological state in agreement with previous independent evidence. The model developed here showed favorable performance that can be used to provide reliable estimates of ecological and environmental state of lakes.     

Quantifying Regional Vegetation Cover Variability in North China during the Holocene: Implications for Climate Feedback

Validating model simulations of vegetation-climate feedback needs information not only on changes in past vegetation types as reconstructed by palynologists, but also on other proxies such as vegetation cover. We present here a quantitative regional vegetation cover reconstruction for North China during the Holocene. The reconstruction was based on 15 high-quality lake sediment profiles selected from 55 published sites in North China, along with their modern remote sensing vegetation index. We used the surface soil pollen percentage to build three pollen-vegetation cover transfer models, and used lake surface sediment pollen data to validate their accuracy. Our results showed that vegetation cover in North China increased slightly before its maximum at 6.5 cal ka BP and has since declined significantly. The vegetation decline since 6.5 cal ka BP has likely induced a regional albedo change and aerosol increase. Further comparison with paleoclimate and paleovegetation dynamics in South China reproduced the regional cooling effect of vegetation cover decline in North China modelled in previous work. Our discussion demonstrates that, instead of reconstructing vegetation type from a single site, reconstructing quantitative regional vegetation cover could offer a broader understanding of regional vegetation-climate feedback.     

沉积硅藻揭示的历史时期水生植被信息以梁子湖为例

水生植被是浅水湖泊生态系统最重要的生态特征之一, 了解其群落历史演化特征, 对生态退化湖泊的修复有着重要指导意义。研究选择长江中下游地区代表性草型湖泊梁子湖, 基于梁子湖沉积岩芯210Pb/137Cs测年、沉积硅藻序列和梁子湖长期水生植被监测记录, 探讨利用沉积硅藻记录来重建该湖历史时期水生植被演替特征的可行性。研究结果表明: 梁子湖沉积硅藻记录对历史时期水生植被的演替有较好的反映; 基于此, 对梁子湖过去近200年的水生植被覆盖度进行了重建; 与湖泊流域历史环境信息的对比分析表明洪水是影响该湖水生植被发育的一个重要因素。研究结果证实了在浅水湖泊中, 沉积硅藻可揭示历史水生植被的信息, 并为该湖的水生植被保护提供科学依据, 同时对该区富营养湖泊的生态修复有重要的指导价值。       

Development of a real-time PCR method for the detection of fossil 16S rDNA fragments of phototrophic sulfur bacteria in the sediments of Lake Cadagno

Lake Cadagno is a crenogenic meromictic lake situated in the southern range of the Swiss Alps characterized by a compact chemocline that has been the object of many ecological studies. The population dynamics of phototrophic sulfur bacteria in the chemocline has been monitored since 1994 with molecular methods such as 16S rRNA gene clone library analysis. To reconstruct paleo-microbial community dynamics, we developed a quantitative real-time PCR methodology for specific detection of 16S rRNA gene sequences of purple and green sulfur bacteria populations from sediment samples. We detected fossil 16S rDNA of nine populations of phototrophic sulfur bacteria down to 9-m sediment depth, corresponding to about 9500 years of the lake's biogeological history. These results provide the first evidence for the presence of 16S rDNA of anoxygenic phototrophic bacteria in Holocene sediments of an alpine meromictic lake and indicate that the water column stratification and the bacterial plume were already present in Lake Cadagno thousands of years ago. The finding of Chlorobium clathratiforme remains in all the samples analyzed shows that this population, identified in the water column only in 2001, was already a part of the lake's biota in the past.     

Limnological studies of Lake Wisdom, a large New Guinea caldera lake with a simple fauna

SUMMARY. Lake Wisdom is of interest because it is large and deep, has oxygen throughout, and yet, apparently as a result of its recent formation, its biota has some surprising lacunae. It is nearly circular and fills the central caldera of Long Island, Papua New Guinea (5° 20′ S, 147° 6′ E). Its maximum length is 13.4 km, and area approximately 95 km². A bathymetric map of the lake to depths of 300 m has been constructed from fathometer transects. Repeated soundings in the deeper parts gave depths of about 360 m. The surface of the lake is approximately 190 m above sea-level and water samples from the deepest part of the lake are fresh, so the basin is apparently sealed. The lake level shows annual fluctuations of c. 1.0 m. Limnological information has been collected during seven visits to Lake Wisdom over the period 1969–76. The surface temperature of the lake was constant at 28°C throughout this period and the temperature falls very gradually to 26–27°C at 60 m, except for a relatively rapid drop of approximately 1°C from 10 to 20 m. One of the most unusual features is the relatively high oxygen concentration in the deepest parts of the lake. Living chironomid larvae and molluscs were collected from the bottom in depths of 360 m. Light penetration in the lake varies greatly depending on the amount of rainfall and the volcanic activity of Motmot, a secondary cone within the lake. The biota of Lake Wisdom is rather simple. There is a low standing crop of phytoplankton. Benthic algae are abundant and diverse although they cover only a limited area because of the depth of the lake. There are no vascular aquatic plants. The pelagic fauna consists of two species of Cladocera and one species of notonectid. There is one species of sponge, four species of molluscs, and a small number of species of aquatic insects including Hemiptera. Odonata, and the larvae of chironomids, mayflies, a caddisfly, and a pyralid moth. Water birds, including ducks, grebes and waders, are fairly numerous. The lake also contains one or more crocodiles.     

The Dry Holocene Megathermal in Inner Mongolia

The paleoclimate since 14 kyr BP (¹⁴C age) was reconstructed based on a 16.22-m-long sediment core collected from Lake Yanhaizi, a saline lake located near the northern limit of the East Asian summer monsoon in Inner Mongolia. Coarse sediments were deposited there during a shrinkage phase of the lake when sand dunes reactivated. These sediments have low organic carbon contents but high maturity indices, indicating that they were deposited in an arid environment. By contrast, based on high organic contents and low maturity indices, fine sediments were deposited during periods of high lake stand in a humid environment. It was in general dry between 8.0 and 4.3 kyr BP. The above dry and wet phases are consistent with those recovered from the arid-semiarid transition zone elsewhere, but are unlike the widely perceived humid Holocene Megathermal reported in east China and the newly reconstructed record in the alpine Retreat Lake in Taiwan. The discrepancy may be due to a relative insensitivity to humidity changes in these two areas since they have both been under the total influence of the summer monsoon. On the other hand, much enhanced evaporation over higher monsoon precipitation at Lake Yanhaizi reduces the effective humidity in the warm climate near the northern boundary of the summer monsoon. This also accounts for the fact the high-temperature Holocene Megathermal, as revealed in the Okinawa Trough and the northern South China Sea, is correlated to the dry phases at Lake Yanhaizi. Conversely, the 4-2-kyr BP coldest period in the Holocene corresponds to a wet phase at Lake Yanhaizi.     

The late-glacial/holocene Bosmina (Eubosmina) fauna of Lake Constance (Untersee) (F.R.G.): traces of introgressive hybridization

Subfossil Bosmina (Eubosmina) remains were analysed in a sediment core from the Untersee of Lake Constance which covered the Late-Glacial/Holocene period.During the Late-Glacial and the early Holocene the lake was inhabited only by Bosmina longispina Leydig. In the Late Holocene a second species, Bosmina coregoni f. kessleri Uljanin, appeared. In the uppermost sediment layers the morphological gap between the two taxa disappeared, apparently from introgressive hybridization.